The present invention relates to switching power supply apparatuses that conduct a pulse frequency modulation control (herein after referred to as a “PFM control”) and a pulse width modulation control (hereinafter referred to as a “PWM control”). Specifically, the invention relates to switching power supply apparatuses, which employ the voltage obtained by rectifying the output from an AC power supply for an input thereof and facilitate reduction of the electric power consumption under a light load and preventing the transformer thereof from buzzing.
Considering the environmental protection, it has been required for the electric and electronic instruments to reduce the electric power consumption thereof. For OA instruments provided with a standby function, it has been demanded to reduce the electric power consumed in the standby mode. Responding to the circumstances described above, it has been required for the power supply apparatuses, which feed electric power to the respective instruments, to reduce the electric power consumption in the standby mode thereof.
A conventional technique for reducing the electric power consumption of the power supply apparatuses in the standby mode thereof employs two power supply apparatuses: a power supply apparatus exhibiting a high output capacity and used in the normal mode of operation and a power supply apparatus used in the stand by mode. However, the employment of the two power supply apparatuses enlarges the occupied volume and increases the manufacturing costs. Therefore, it is difficult to obtain inexpensive and practical products by employing the two power supply apparatuses.
Conventional techniques for reducing the electric power consumption using only one power supply apparatus employ a switching power supply apparatus and lower the driving frequency (switching frequency) of the power MOSFET used for a switching device in the standby mode of operation. By one of the conventional techniques, the switching frequency is changed over to the lower one as the load current exceeds the reference value to the smaller side. (The switching frequency changes discontinuously between two frequencies). By the other conventional technique, the switching frequency is lowered in response to the load current smaller than the reference value. (The switching frequency changes continuously in response to the load current.)
FIG. 5 is a block circuit diagram of a conventional switching power supply apparatus.
In FIG. 5, the AC input from AC power supply AP1 is full-wave rectified by diode stack DS1 and DC voltage Vin obtained by smoothing the full-wave rectified voltage by capacitor C1 is fed to primary winding N1 of transformer T. MOS transistor Q1 as a switching device is connected in series to primary winding N1. Power MOS transistor Q1 conducts ON and OFF operations in response to the driving signal fed from switching control circuit 100 integrated in to an IC. In response to the ON and OFF of MOS transistor Q1, a pulsating voltage is generated across secondary winding N2 of transformer T. The pulsating voltage is rectified by diode D1 and output voltage Vout obtained by smoothing the rectified voltage by capacitor C2 is fed to load 200.
Output voltage Vout fed to load 200 is divided and detected by resistors R1, R2 and the detected value is compared with a reference voltage (not shown) in shunt regulator SR1. The result of the comparison is fed to feedback terminal FB of switching control circuit 100 as feedback signal VFB via photocoupler PC1. In other words, the current corresponding to an error signal obtained by amplifying the difference between the divide voltage value, obtained by dividing output voltage Vout by resistors R1, R2, and the reference voltage, flows through light emitting diode LED of photocoupler PC1. The light corresponding to the current flowing through light emitting diode LED impinges on phototransistor PT of photocoupler PC1. The current corresponding to the impinging light amount flows through phototransistor PT of photocoupler PC1. Since the output (collector) of phototransistor PT is pulled up by resistor R4 in switching control circuit 100, feed back signal VFB fed to feedback terminal FB becomes smaller (closer to ground potential GND) as the current flowing through phototransistor PT is larger.
Due to the circuit configuration described above, a larger current flows through phototransistor PT, and smaller feedback signal VFB is obtained as output voltage Vout becomes larger than the target voltage set by the reference voltage described above. In contrast, a smaller current flows through phototransistor PT, and larger feedback signal VFB is obtained as output voltage Vout becomes smaller than the target voltage set by the reference voltage described above. Since the current fed from the switching power supply apparatus to capacitor C2 tends to be more excessive than the current consumed in load 200 as load 200 becomes lighter, output voltage Vout becomes larger than the target voltage. Therefore, feedback signal VFB may be deemed as a load signal that indicates the load (the magnitude of the current consumed in load 200).
As a current flows through primary winding N1 of transformer T, a voltage is generated across auxiliary winding N3 of transformer T. The voltage generated across auxiliary winding N3 is rectified by diode D2, smoothed by capacitor C3 and fed to power supply terminal Vcc of switching control circuit 100.
It is impossible to feed electric power from auxiliary winding N3, when the switching operation of the switching power supply apparatus is not conducted such as at the start of the switching power supply apparatus. When it is impossible to feed electric power from auxiliary winding N3, electric power is fed directly from DC voltage Vin via VH terminal VH of switching control circuit 100 and starter circuit 101. In other words, starter circuit 101 receives energy feed from DC voltage Vin and feeds a charging current to capacitor C3 via power supply terminal Vcc. As the voltage at power supply terminal Vcc reaches a predetermined voltage, starter circuit 101 stops feeding the charging current.
Reference voltage generator circuit 102 is connected to power supply terminal Vcc. Reference voltage generator circuit 102 generates a reference voltage of 5 V. (Hereinafter, the reference voltage of 5 V will be referred to as “reference voltage 5V”.) Reference voltage generator circuit 102 feeds reference voltage 5V to resistor R4 and the relevant circuits in switching control circuit 100.
The voltage across sensing resistor Rs, that is the detection signal of the current flowing through power MOS transistor Q1, is fed to current sensing terminal IS of switching control circuit 100. Ground terminal GND of switching control circuit 100 is shown in FIG. 5.
For keeping the ordinary output voltage at a certain value, the switching power supply apparatus described above monitors the output voltage, feeds back the output voltage data to the switching control circuit that drives the switching device, conducts a PWM control for adjusting the pulse width of the switching device (negative feedback control), and outputs from output terminal OUT a signal for conducting the ON-OFF drive of power MOS transistor Q1.
The switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2004-304885 (Patent Document 1) judges the load, conducts a PWM control when the load is greater than a predetermined value, and shifts the switching frequency to the lower side under a light load as the load becomes closer to zero, and further elongates the pulse width of the switching device gradually.
The switching power supply apparatus disclosed in the Patent Document 1 also changes the ON-pulse width of the switching device thereof based on DC voltage Vin under a light load and shortens the ON-pulse width, when DC voltage Vin is high, for preventing the switching power supply apparatus from buzzing.
The buzzing of the switching power supply apparatus and the countermeasures for preventing the buzzing from causing will be described below with reference to FIGS. 6 and 7.
When the magnetization energy of transformer T is large, that is when the load current flowing through load 200 is large to some extent, buzzing is caused by the switching frequency entering the audible frequency range. On the other hand, it is preferable to shift the switching frequency to the lower side under a light load as described above. Therefore, the switching frequency is set to exhibit the characteristics as described in FIG. 6.
FIG. 6 describes the switching frequency characteristics set to be optimum for the AC input voltage of 100 Vac. In FIG. 6, the horizontal axis represents the load value (in detail, a signal indicating the load current or feedback signal VFB described above) and the vertical axis represents the switching frequency. In the region designated as “Standby electric power characteristics impaired”, the switching frequency is high even though the load is light. Therefore, the electric power conversion efficiency of the switching device is impaired and the standby electric power characteristics are impaired in the region designated as “Standby electric power characteristics impaired”. In the region designated as “Buzzing”, the switching frequency coincides with the audible frequency and the magnetization energy of transformer T is high enough to cause buzzing. In FIG. 6, the unfavorable regions described above are avoided so that the standby electric power characteristics may be prevented from being impaired and buzzing may be prevented from causing at the AC input voltage of 100 Vac.
However, as the AC input voltage is changed from 100 Vac to 200 Vac while the ON-pulse width of the switching device is unchanged, the switching frequency starts decreasing from a load value greater than the load value where the switching frequency starts decreasing in a case of the AC input voltage of 100 Vac. As a result, the switching frequency changes through the buzzing range. In short, buzzing is caused at the AC input voltage of 200 Vac.
For preventing the buzzing from causing at the AC input voltage of 200 Vac, the switching power supply apparatus disclosed in the Patent Document 1 changes the ON-pulse width of the switching device under a light load based on DC voltage Vin. By the technique described above, the ON-pulse width at the AC input voltage of 200 Vac is shortened so that the switching frequency may not be changed between the AC input voltages of 100 Vac and 200 Vac, as shown in FIG. 7.
The switching power supply apparatus disclosed in the Patent Document 1 gradually elongates the ON-pulse width of the switching device thereof under the light load, as the load becomes further lighter. As the load becomes lighter, the ON-period for one ON-state of power MOS transistor Q1 becomes longer, causing larger pulsation of output voltage Vout. Since the ON-period becomes longer as the load becomes lighter, the OFF-period becomes also very long, making the frequency extremely low. The extremely low frequency further causes response delay, when the load becomes heavy suddenly when the main switch of the load shifts from the OFF-state thereof to the ON-state thereof.
In the switching power supply apparatus disclosed in the Patent Document 1, it is assumed that DC voltage Vin is smoothed almost perfectly to be a constant voltage (cf. FIG. 3 in the Patent Document 1). Therefore, the switching power supply apparatus disclosed in the Patent Document 1 is not applicable to a switching power supply apparatus for improving the power factor (hereinafter referred to as a “PFC power supply apparatus”). For improving the power factor, it is necessary for the PFC power supply apparatus to feed a signal including the phase data of an AC power supply to the switching control circuit thereof. Therefore, a capacitor, the capacitance of which is small enough only to remove the ripples caused by the switching operation, is used for capacitor C1 shown in FIG. 5. DC voltage Vin has a positive pulsating waveform as described with reference to FIGS. 2(a), 2(b) later. It is impossible for the switching power supply apparatus disclosed in the Patent Document 1 to handle such a waveform as described above and to judge the value of DC voltage Vin.
The Patent Document 1 does not describe anything about the protection against overload including short-circuiting or the countermeasures against the overcurrent caused at the start of the switching power supply apparatus.
In view of the foregoing, it is an object of the invention to overcome the problems described above, and to provide a switching power supply apparatus that facilitates the reduction of the power consumption in the standby mode and prevents buzzing.
It is another object of the invention to provide a switching power supply apparatus that facilitates quick response to the change over from the no-load condition to the normal load condition, thereby preventing an overcurrent from causing at the start thereof, and protecting against short-circuiting.
It is a further object to provide a switching power supply apparatus applicable to a PFC power supply apparatus.
Further objects and advantages of the invention will be apparent from the following description of the invention.